The invention relates to an optical system for aligning two patterns using a third pattern or intermediate reference pattern, and in particular to a photorepeater using this optical alignment system for positioning a semiconductor wafer along two reference axes.
The manufacture of integrated circuits involves the formation on a substrate of windows for locating the implantation or the treatment. This substrate is covered with a photosensitive resin layer. The formation of windows is achieved by masking this resin with a mask carried by a reticle. Previously, direct contact or proximity was used as the method of transfer. Present processes use the technique of transfer by optical projection.
This projection may be effected in the ratio 1/1, and the mask is wholly projected onto the wafer. This projection may also be effected by division of the image: either by analysis of the mask through a mobile slit, or by using the photorepetition technique in the ratio 1/n.
In the technique for manufacturing circuits by direct photorepetition, each pattern to be formed is projected directly onto the semiconductor wafer previously covered with a layer of photosensitive resin, in accordance with a pre-established program, the position of the wafer being controlled by interferometry along two directions X and Y. Conventionally, photorepetition is carried out by offsetting the wafer in two directions X and Y orthogonal with respect to each other.
In all cases it is necessary to position, with respect to each other and very accurately, the reticle and the wafer of semiconductor material on which the integrated circuit is to be formed. For this numerous alignment processes have been proposed. In general, use is made of additional patterns comprising alignment reference marks carried by the reticle and by the semiconductor wafer. Depending on the process used, the number of these reference marks, their arrangement or their configuration must be such that they allow alignment along two reference axes X, Y and possibly angular alignment of the semiconductor wafer. Two approaches may be essentially distinguished for obtaining the desired alignment.
According to a first approach, the alignment is achieved by bringing into direct coincidence the alignment reference marks carried by the reticle and the semiconductor wafer or the projection thereof. This first approach raises a certain number of difficulties which will be discussed hereafter. It is well-known that resin is sensitive to a narrow range of wavelengths. The source used for exposure of the pattern to be projected onto the semiconductor wafer, i.e. during exposure of the resin, is either a narrow-spectrum source, as for example a laser, or a wider-spectrum source associated with filters. If a single source is used for the operations of alignment and exposure of the resin, or if more generally two separate sources having the same wavelength are used, care must be taken that the alignment reference marks and the patterns to be projected are illuminated solely during the phases concerned, i.e. respectively during the alignment phase and the exposure phase. It is then necessary to use accurately positioned masks, since the alignment reference marks and the pattern to be projected are very close to each other. Furthermore, if the system for illuminating the alignment reference marks is distinct from that for the exposure, the first system must be retracted during the exposure phase. If an alignment wavelength is used distinct from that of the exposure, lenses or more generally a correction system must then be introduced during the alignment phase, with great positioning accuracy. These lenses or this correction system must also be retracted during the exposure phase. It is furthermore necessary to switch the detection system(s) required for the alignment, this system being for example positioned in the pupil of the objective used for the exposure.
In a second approach, intermediate alignment reference marks are used. With this approach, the alignment, one with respect to the other, of the reticle and of the semiconductor wafer is no longer achieved directly by bringing alignment reference marks or the projection thereof into coincidence, but sequentially by use of intermediate alignment reference marks. Although removing all or part of the difficulties which have just been recalled and which are specific to the first approach, the processes of the known art according to this second approach are complex and multiply the number of intermediate steps required for obtaining the desired alignment. The multiplication of the number of steps, in particular, introduces correlatively the multiplication of the risk of errors and reduces the accuracy of the alignment.